Faults in an AC power supply may exist as a phase loss, an under-voltage condition, an over-voltage condition, or other condition. Such faults may result from a variety of reasons including breaker trips, fuse loss, inverter faults, shorted turns in a transformer, and ground faults. Frequency or phase errors may also occur when the AC power frequency falls outside of specified ranges. Power supply faults have deleterious impact on electrical equipment, and in many applications cannot be tolerated.
Achieving reliability by using multiple power sources to supply a single-phase or DC load device is relatively simple. Multiple sources, for example, can be connected in parallel with the load device so that a failure of one source will not shut down the load. For single-phase AC supplies, this works fine, provided the phases of the respective sources are, maintained in substantial synchronism. Other precautions, however, must be taken if the difference between the respective phases become significant. Connecting a single-phase AC load device to multiple single-phase AC sources does not require precise phase alignment because most load devices can tolerate minor phase differences. Using two or more multiphase AC sources to power a multiphase load, however, presents certain challenges. While it is possible to connect multiphase sources in parallel, a failure of any phase usually results in a failure in another phase or in the other source. Furthermore, prior systems cannot handle multiphase AC power sources in parallel to supply high performance loads where high reliability is essential.